The retina carries out the first steps in the process of "seeing." Much of the power of the retina to accomplish this task lies in the synapses, by means of which retinal neurons communicate with each other. This proposal describes experiments to investigate the normal working of retinal synapses, concentrating in particular, on the first synapse of the visual system: the synapse between photoreceptors and bipolar cells. Two sets of questions are addressed here. A first set of questions concerns the way in which retinal cells release neurotransmitter. Cultures of chick retinal cells derived from early chick embryos will be used in this part of the study. Electrophysiological methods will be used to discover the way in which transmission at these synapses differs from synaptic transmission elsewhere in the nervous system. A second set of questions deals with the way in which the probable neurotransmitter of photoreceptors (glutamic acid) opens or closes channel molecules on bipolar cell membranes. To answer these questions we will use bipolar cells isolated from the salamander retina. The behavior of individual channel molecules will be observed using the "patch-clamp" method to measure ion currents flowing through single membrane channels.